J Solid State Electrochem (2000) 4: 225±230

Ó Springer-Verlag 2000

ORIGINAL PAPER

W.F. Zhang á P. Schmidt-Zhang á G. Koûmehl

Analysis of photocurrent spectra at polybithienyl ®lm coated on Pt

Received: 6 January 1999 / Accepted: 6 June 1999

Abstract Photoelectrochemical measurements have been performed at a polybithienyl (PBT) ®lm (doping level of 1 ´ 1018/cm3) deposited on a platinum electrode. The cathodic photocurrents and negative slope of the Mott-Schottky plot indicate that the PBT ®lm has the features of a p-type semiconductor. The cathodic photocurrents are interpreted in terms of the GaertnerButler model on the basis of the theory of the semiconductor|solution interface. The (iphhm)2/n vs. hm plots taken from the photocurrent spectra show two linearities for n ˆ 1 in the wavelength range from 460 nm to 490 nm and for n ˆ 4 in the wavelength range k > 490 nm. The band gaps of the PBT ®lm were determined to be 2.05 ‹ 0.05 eV for n ˆ 1 and 1.55 ‹ 0.05 eV for n ˆ 4. The ¯at-band potential is 0.33 V (vs SCE). From the slope of the Mott-Schottky plot at the modulation frequency of 3 kHz, the dielectric constant e of the ®lm and the thickness of the depletion layer W0 of the PBT ®lm were determined to be 7.4 and 0.29 lm, respectively.

photoelectrochemical behaviour of ECOPs is signi®cant. In the last decade, much more work in this ®eld has been reported [1±9]. Most reports are about poly-3-methylthiophene (PMT), but few about polybithienyl (PBT) [5±7, 9]. Recently, Plieth and co-workers [8] and Semenikhin et al. [9] have reported the photoelectrochemical behaviour of polybithienyl (or polybithiophene) (PBT) in aqueous and non-aqueous solutions, respectively. However, there is a lack of the analysis in detail. Therefore, we have tried to analyse the photocurrent spectra of PBT using the Gaertner-Butler model [10, 11] on the basis of an assumption that the PBT ®lm is a quasi-semiconductor and the behaviour of the PBT|solution interface corresponds with the theoretical model of the semiconductor|electrolyte interface as suggested in the literature [12, 13]. The results of this analysis are surprisingly good. In the present paper we report these results and attempt to interpret them.

Key words Polybithienyl á Organic semiconductor á Photocurrent á Gaertner-Butler model á Band gap energy

Experimental

Introduction The photoelectrochemical properties of electric conducting organic polymers (ECOPs) are very interesting owing to their applications in solar energy conversion and optical displays. Therefore, investigation of the

W.F. Zhang (&) á G. Koûmehl Institut fuÈr Organische Chemie, Freie UniversitaÈt Berlin, Takustrasse 3, D-14195 Berlin, Germany P. Schmidt-Zhang Forschungszentrum Sensorik Greifswald eV, Brandteichstrasse 19, D-17489 Greifswald, Germany

Preparation of the PBT ®lms The equipment employed for the electrochemical synthesis of PBT ®lm is described in detail in a previous paper [14]. The PBT ®lm for the photocurrent measurements was galvanostatically synthesized at a current density of 1 mA cm)2 on Pt disc electrodes (0.071 cm2) in a propylene carbonate +0.1 M LiClO4 + 0.01 M BT solution under nitrogen atmosphere at room temperature. The thickness of the PBT ®lm was determined by interferometry to be 1.3 lm [15]. It is assumed that the ®lm thickness is independent of the doping level dPBT ˆ 2Qox/Qgr ˆ 0.2% where Qox is the charge for PBT oxidation and Qgr is the charge needed for chain growth of the PBT [14, 15]. The anion doping concentration (or PBT cation concentration) is NA ˆ 1 ´ 1018/cm)3, de®ned as follows: NA ˆ

K VPBT

ˆ

dPBT VVPBT BT VPBT

ˆ

dPBT VBT

…1†

where K is the charge number of the oxidized PBT, …BT†K‡ m , and VPBT and VBT are the total volume of the doped PBT ®lm and BT monomer unit volume, respectively. Since the thiophene oligomers are subject to a strong rotational disorder instead of a planar one [16], it is assumed that the BT monomer unit is a cylinder with a

226 diameter of 0.85 nm and a length of 1.7 nm which is double the distance between two 2,5-thienylene units of a neutral polythiophene molecule [17]. Before and after the photoelectrochemical measurements the stability of the PBT ®lms was tested by means of cyclic voltammetry at a potential range of )1.0 V to 0.6 V vs. SCE in a deoxygenated 0.2 M LiClO4 aqueous solution. The cyclic voltammograms obtained are peakless and prove that the PBT ®lm is electrochemically stable in this aqueous electrolyte solution [15].

Photocurrent measurement A potentiostat/galvanostat (EG&G model 273) was used to control the electrode potential in photoelectrochemical measurements, which were performed in a three-electrode photoelectrochemical cell with a quartz window. A monochromator with a wavelength range of 250±1500 nm and a light chopper working at a modulation frequency of 0.1 Hz were used. An argon plasma lamp, operating at 300 W, served as the light source. The photocurrent was ampli®ed by a two-phase lock-in ampli®er (EG & G model 278) and recorded by a x-y recorder (Linseis). The photoelectrochemical measurements were performed at several potential points in the range of )1.0 V to 0.6 V (vs SCE) in 0.2 M LiClO4 aqueous solution.

Results and discussion The working electrode for the photoelectrochemical measurements was a PBT ®lm (d ˆ 1.3 lm, dPBT ˆ 0.2%, NA ˆ 1 ´ 1018/cm)3) coated on a Pt disc electrode (0.071 cm2). The PBT ®lms synthesized by the galvanostatic method have a regular morphology and compact structure which can be seen from the scanning electron microscopy (SEM) diagram in Fig. 1. Fig. 1 Scanning electron microscopy diagram of a PBT ®lm formed by galvanostatic synthesis

The photocurrent spectra were measured at several constant electrode potentials of 0.6, 0.5, 0.4, 0.3, 0.2, 0.0, )0.2, )0.4, )0.6, )0.8 and )1.0 V. At the ®rst three potentials no photocurrent was observed. The cathodic photocurrent spectra of the PBT on Pt in 0.2 M LiClO4 aqueous solution obtained at di€erent electrode potentials are shown in Fig. 2. Each photocurrent spectrum exhibits a current maximum which is similar to that of the absorption spectra of PBT ®lms in oxidized and reduced forms at almost the same wavelength [18] (Fig. 3). Photocurrent spectra such as these with a maximum depending on the electrode potential (E), were obtained on PMT in an aqueous electrolyte [19] and are similar to the photocurrent spectra of a passive layer on Sn called an amorphous semiconductor [13]. On such materials there is no sharp band edge as in a crystalline semiconductor. The mobility of the electrons and holes in these materials follows a hopping mechanism due to energy distribution and delocalization [13]. The maxima of the photocurrent spectra at E ³ )0.2 V appear at the same wavelength of 455 nm (kmax) which shifts slowly to a longer wavelength with the potential negative change until kmax ˆ 467 nm at E ˆ )1.0 V (Fig. 2). This kmax shift implies a decrease of the transition energy between HOMO and LUMO in the PBT molecules. At E ˆ 0.0 V and )0.2 V, the photocurrent spectra show a considerable photocurrent shoulder in the neighbourhood of 550 nm. With the negative change of the potential, this shoulder changes gradually and disappears at )0.4 V after which the photocurrent spectra are almost symmetrical. This photocurrent shoulder could be considered as the overlapped band gap edge as described in the conventional model of the semiconductor|solution interface, because of a linearity of the

227

Fig. 4 Potential dependence of the cathodic photocurrents at k ˆ 460 nm, taken from the photocurrent spectra in Fig. 2

to that of PBT obtained under the potential sweep [8] and similar to that of PMT in aqueous solutions [20]. The photocurrent spectra in Fig. 2 are analysed using the Gaertner-Butler model [10, 11] on the basis of an assumption that the PBT ®lm remains unchanged under the illumination and behaves as a quasi-semiconductor. In this case, the photocurrent density (iph) is a function of the total photon ¯ux (F0), monochromatic absorption coe€cient (a) and thickness of the depletion layer (W), written as   …2† iph …m†  f e; Lp ; U0 …m†; a…m†; W …E† Fig. 2 Cathodic photocurrent spectra of PBT ®lm at the potentials: 1 0.3 V, 2 0.0 V, 3 )0.2 V, 4 )0.4 V, 5 )0.6 V, 6 )0.8 V, 7 )1.0 V in H2O + 0.2 M LiClO4 solution (pH 4.8)

Fig. 3 Photocurrent spectrum of PBT at E ˆ 0.0 V (curve 1) taken from Fig. 2 (no. 2) in comparison with visible absorption spectra of the oxidized and neutral PBT forms (curves 2, 3) taken from [18]

Mott-Shottky plot and also photocurrent behaviour in this potential range as mentioned in the following text. The potential dependence of the photocurrent is shown in Fig. 4 where the photocurrent shows a plateau at E > )0.4 V and then rises. This behaviour is identical

where e, m and Lp are elementary charge, wavenumber and di€usion length of minority carriers, respectively. If aLp  1 and aW  1, according to the equation ÿ
2=n ˆ …eU0 WAn †2=n …hm ÿ Eg † …3† iph hm the (iphhm)2/n vs hm plots should be linear. Here h and Eg are respectively Planck's constant and the band gap energy, An is a constant essentially representing the electronic transition probability and is independent of the illumination; n ˆ 1 represents a direct electronic transition and n ˆ 4 holds for an indirect transition. In the wavelength range from 460 nm to 490 nm, the (iphhm)2 vs. hm plots are linear (Fig. 5A), which indicates a direct electron transition and is similar to the photoelectrochemical behaviour of lead oxide thin ®lms [21]. The intercepts of the hm axis give an energy value of 2.05 ‹ 0.05 eV which agrees with the value of the band gap energy of PMT given in the literature [19, 20, 22]. The slopes of these plots, (eF0WA1)2, depend on the electrode potentials because W is a function of the electrode potential E, expressed as   kT 1=2 …4† W ˆ W0 Efb ÿ E ÿ e where W0 is the thickness of the depletion layer at the potential di€erence Efb ) E ) kT/e ˆ 1 V and Efb, k and T are ¯at-band potential, Boltzmann constant and absolute temperature, respectively. In a small potential region, E ³ )0.2 V, the (eF0WA1)2/E plot exhibits a

228

Fig. 5 A (iphhm)2 vs hm plots; B (iphhm)1/2 vs hm plots of the PBT ®lm. The data for these plots are taken from the photocurrent spectra in Fig. 2

linearity and intersects the E axis at 0.3 V (Fig. 6A) in agreement with that obtained from the iph2/E plot (Fig. 7A) and the Mott-Schottky plot (Fig. 7B) in the same potential region corresponding to the equations   kT 2 2 …5† iph ˆ …eU0 aW0 † Efb ÿ E ÿ e and

  1 2 kT ˆ E ÿ E ÿ fb 2 Csc ee0 eNA e

…6†

where Csc, e, and e0 are the capacity of the space charge layer, the semiconductor dielectric constant and the vacuum dielectric constant. The ¯at-band potential is determined from the intersects of the Mott-Schottky plot and iph2/E plots at the E axis, being 0.33 V vs. SCE [15]. This value is near to the ¯at-band potential of PMT (Efb ˆ 0.35 V vs. SCE) in aqueous solution [20]. From the slope of the Mott-Schottky plot the dielectric constant of PBT ®lm is evaluated to be e ˆ 7.4. Inserting the parameters e, e0, e and NA in the equation  1=2 2 …7† W0 ˆ ee0 eNA gives W0 ˆ 0.29 lm. Using Eq. 4, the parameter W is then estimated for PBT ®lm at di€erent electrode

Fig. 6 Potential dependence of slopes of: A (iphhm)2/hm plots and B (iphhm)1/2/hm plots, obtained from the photocurrent spectra in Fig. 2

potentials (Table 1). As mentioned above, in a potential region E ³ )0.2 V the photoelectrochemical behaviour of the PBT ®lm accords well with Eqs. 5 and 6, which completely describe the behaviour of photocurrents in a Mott-Schottky junction at the semiconductor|solution interface. The linearity behaviour of the Mott-Schottky plot for ECOPs is little reported in the literature except for [8] by reason of the irregular morphology and structure of the ECOPs. Generally, The ECOPs are regarded as porous electrodes because of their rough morphology and porous structure [18]. However, the PBT ®lm reported in this paper has a regular morphology and compact structure like the crystal lattices in an inorganic semiconductor (see Fig. 1), which is an important prerequisite to form the Mott-Shottky junction at the semiconductor|solution interface. Nevertheless, the surface state (or structure) of the PBT ®lm depends on many factors, for example the electrode potential which could change the surface state of the PBT ®lm leading to a distinction of the photocurrent behaviour at E < )0.2 V from that at E ³ )0.2 V where the Gaertner model is valid (Figs. 5±7). In the wavelength range k > 490 nm, the (iphhm)1/2 vs hm plots are linear for n ˆ 4 (indirect transition) (Fig. 5B), which corresponds to an absorption peak in the absorption spectra of the PBT ®lms at 700 nm < k < 900 nm in Fig. 3 [18]. The intercepts of the hm axis give an energy value of 1.55 ‹ 0.05 eV which could represent the transition from valence band to the upper bipolaron state being inside the band gap

229

According to the summary in the literature [16], the band gap of the aromatic ECOPs is determined by ®ve factors: (1) the energy related to the degree of bond length alternation Edr, (2) the mean deviation from planarity of the polymer molecule E°, (3) the aromatic resonance energy of the cycle Eres, (4) the substituent e€ect Esub and (5) intermolecular or interchain coupling in the solid state Eint. From this one can easily understand the variance of the photocurrent behaviour of the PBT ®lm. Esub does not exist in PBT, but the other four factors are all related to the electric ®eld, i.e. the electrode potential. Among these, Eres is related to the p-conjugated system and plays a main role in the ®nal value of Eg. Obviously, the negative potential is favoured for the delocalization length or con®nement length in the aromatic p-conjugated system leading to a decrease of the band gap in the PBT ®lm.

Conclusion

Fig. 7 A iph2 vs E plot at wavelengths of 380, 460, 550 and 600 nm obtained from the photocurrent spectra in Fig. 2; B Mott-Schottky plot of PBT ®lm in H2O + 0.2 M LiClO4 solution at a frequency of 3 kHz Table 1 Thicknesses W of the depletion layer of PBT ®lm on Pt in H2O + 0.2 M LiClO4 solution at di€erent electrode potentials E E (V) W (lm)

)1.0 )0.8 )0.6 )0.4 )0.2 0.0 0.33 0.30 0.28 0.24 0.21 0.17

0.3 0.02

between the valence band and the conduction band, such as that in polythiophene (PT) [23]. The slopes of (iphhm)1/2 vs hm plots are proportional to the electrode potential, i.e. the (eF0WA4)1/2/E plot is linear over the whole potential range (Fig. 6B). Its extension intersects the E axis at 0.63 V more positive than the ¯at-band potential. As mentioned above, at E < )0.2 V the photocurrent behaviour deviates from linearity and the obtained band gap energies decrease with the negative potential change, especially at E ˆ )1.0 V (see Fig. 5). These phenomena cannot be interpreted by the common theory of the semiconductor because of the special character of the PBT ®lm for example, the one dimensional conducting system, polaron and dipolaron charge carriers, the p-conjugated electrons in the aromatic ring and the mesomeric forms have di€erent band gaps, which can be strongly in¯uenced by the electric ®eld. However, the known Franz-Keldysh e€ect given by DE ˆ 2.8 ´ 10)7 F2/3 [24], i.e. electric ®eld in¯uence on the band gap, is here very small (DE ˆ 0.007 eV) and hence negligible.

The cathodic photocurrents and the Mott-Schottky plot with a negative slope indicate that PBT ®lm has features of a p-type semiconductor. By a detailed analysis of the photocurrent spectra and Mott-Schottky plot, we have obtained some important parameters such as Eg, Efb, e, W0 and W as well as other important information on the PBT ®lm. The theories of conventional semiconductor electrochemistry and photoelectrochemistry can be applied to interpret the photocurrent behaviour of the organic semiconductor ECOPs. The validity of this application depends strongly on the morphology and structure of the ECOPs. In fact, PBT is a mixture of various chain lengths [14, 15], and the ratio of the oligomers to polymer depends strongly on the conduction of the PBT formation. This can make the photoelectrochemical behaviour of PBT more complicated and dif®cult to understand than that of the conventional semiconductor. Acknowledgements The authors are greatly indebted to Professor W. Plieth (Technical University Dresden) for the opportunity to work in his laboratory and to Dr. Ghobarkar (Free University of Berlin) for the SEM diagram.

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